This invention relates to a matrix display device comprising a row and column array of electro-optical picture elements defined by opposing electrodes with an electro-optical display medium disposed therebetween, and a switching transistor for each picture element to which switching and data signals are supplied from driver means via a switching signal conductor and a data signal conductor.
A matrix display device of this kind is suitable for displaying alpha-numeric or video (for example, TV) information using passive electro-optical display material such as liquid crystal material, electrophoretic suspensions and electrochromic materials, or light-emitting, for example electroluminescent, display material. Such active matrix addressed display device may typically consist of a matrix array of a very large number, for example around 200,000 or more, picture elements.
In a known example of such a matrix display device in which the electro-optical medium comprises liquid crystal material, each picture element is defined by a respective electrode carried on one substrate and a facing portion of an electrode common to all picture elements carried on another substrate. The switching elements comprise thin film transistors (TFTs), having substantially identical operational characteristics and which are arranged adjacent their associated picture element electrodes on the one substrate in an array of row and columns with the drain of each TFT being connected to its associated picture element electrode. The sources of all TFTs associated with one column of picture elements are connected to a respective one of a set of parallel column (data signal) conductors and the gates of all TFTs associated with one row of picture elements are connected to a respective one of a set of parallel row (switching signal) conductors extending at right angles to the set of column conductors. The row conductors are repetitively scanned in sequential fashion with a switching signal so as to turn "on" all TFTs in each row in turn and data signals, derived for example by sampling the corresponding TV line signal in the case of TV display, are applied to the column conductors as appropriate for each row in turn to build up a display picture. When the TFTs are turned "on", the picture element electrodes of the row concerned charge up to the applied column (data) voltage. When the switching signal disappears the TFTs are turned "off", thus isolating the picture element electrodes of the row so that charge is stored on each picture element. The picture elements stay in the state in to which they were driven, until the next time the TFTs are addressed, this usually being in the subsequent field period.
It will be appreciated that with the above-described display device the minimum number of rows and column conductors necessary corresponds to the number of rows and columns respectively of picture elements. In addition to the need therefore to devote a proportion of the area of the display device to the accommodation of the row and column conductors, there is also a possibility that in view of the large number of conductors involved one or more could prove defective, perhaps rendering the device unusable. Obviously, the more conductors employed, the greater this possibility becomes so that with comparatively large area display devices yields can be seriously effected.
Furthermore, the numbers of row and column conductors necessary can also cause difficulties with the production of high resolution, small area display devices such as small area LC display devices intended for use in a projection system. For such a purpose a small area LC display device having a very compact array of picture elements is required to provide sufficient picture element density for adequate resolution after projection. In conventional LC display devices the provision of a compact array of picture elements is hindered by the numbers of row and column conductors needed and the area they occupy.
It is an object of the present invention to provide an active matrix addressed display device in which fewer address conductors need be provided.